This invention relates to rotational molding and more particularly to an improved method for forming leak-proof connections to hollow articles rotationally molded from a synthetic resinous material.
Various processes used for commercially molding articles from synthetic resinous materials include, for example, blow molding, injection molding and rotational molding. Each specific molding application must be investigated to determine the best process. Rotational molding is useful for producing hollow articles of a very uniform wall thickness. If the mold is shaped to allow contact between the inner mold surfaces and the resinous material and the inner mold surfaces are heated uniformly, then the walls of the finished article will be uniform to within plus or minus five percent of the desired value.
Rotational molding generally involves heating of a charged mold while simultaneously rotating biaxially or about two different axes until, in the case of a thermoplastic resinous material, the resinous material melts. As the mold is rotated biaxially, the resinous material tends to puddle or flow to the lowest point in the mold cavity and gradually coats the interior walls or surfaces of the mold cavity. The mold is then cooled to cure the resin while continuing the biaxial rotation. After the mold is cooled, the molded article is removed from the mold cavity and the mold is recharged with raw material for molding the next article. The molded article will have exterior surfaces which are excellent reproductions of the interior surfaces of the mold cavity. Typical apparatus for rotationally molding articles is shown, for example, in U.S. Pat. No. 2,957,202 which issued Oct. 25, 1960 to Rekettye. Other common types of apparatus are also described, for example, in the 1968 Modern Plastics Encyclopedia, Vol. 45, No. 1A, September 1967, at page 825, and in the 1969-1970 Modern Plastics Encyclopedia, Vol. 46, No. 10A, October 1969, at page 568.
Rotational molding is commonly used for manufacturing tanks and other hollow articles from synthetic resinous materials because a hollow, single-piece tank can be easily manufactured with uniform walls. Other molding processes may require that a hollow tank be manufactured as two or more separate shells which are bonded together. This necessitates extra manufacturing steps and the finished tank will have a seam which may leak or may be weaker than the remainder of the tank. Furthermore, the cost of the molding machinery and of the molds is generally more for injection molding and blow molding than for rotational molding. However, labor costs for operating rotational molding equipment may be higher than for other processes. Also, some difficulty has occurred in the past when attaching leak-proof fittings to rotationally molded articles such as fuel tanks. In the past, fuel tanks have been rotationally molded from relatively hard synthetic resins, such as nylon, which is then drilled and tapped for receiving a threaded fitting or connector. Tanks manufactured in this manner are expensive due to the cost of the nylon and also threads in the nylon are easily stripped if the fittings are not carefully attached, resulting in leakage. Less expensive, softer resins, such as a polyethylene, have been unsatisfactory because threaded fittings do not hold in the soft tank walls and other methods for making leak-proof connections to the soft tank walls were unsatisfactory.